An important step in the fabrication of integrated circuits is the use of masks on which circuit patterns have been formed by any one of a number of techniques. The circuit pattern is then projected onto a silicon wafer containing a photosensitive coating e.g. photoresist by the use of radiation such as ultraviolet light or X-rays. Since X-rays have substantially shorter wavelengths than ultraviolet light, X-ray lithography permits much finer pattern line discrimination resulting in the more efficient utilization of space on the wafer.
Masks used in X-ray lithography need not be optically transparent because X-rays are transmissable through optically opaque material. Thus, the mask may be made of optically opaque materials such as metals. An advantage of metals as mask substrates is their relative resistance to dimensional change as compared to, e.g., plastics.
The circuit pattern formed on the X-ray mask may consist of an X-ray absorbing material such as gold deposited in the shape of that pattern (positive mask) or as nonabsorbing pattern shapes imbedded in a field of absorbing material (negative mask). For either positive or negative X-ray circuit masks a necessary step is the deposition of a layer of absorbing material, e.g., gold. One such method for coating a metal substrate with gold or similar metal is by the well known technique of electroplating wherein the substrate which is made of a conductive metal is physically connected to the cathode.
Practical X-ray lithography systems employ so-called soft X-rays with low penetration power necessitating very thin mask substrates. Since the substrate which may be of the order of 1 .mu.m thick and several inches in diameter is fragile and easily damaged, achievement of good coating without physical contact between the substrate and cathode is highly desirable.
The present invention relates to an apparatus and method for electroplating a mask substrate with gold or similar X-ray absorbing metal without any potentially damaging contact between the substrate and the solid electrode comprising the cathode.